The WHERE Clause

The WHERE clause selects a subset of the rows in the intermediate result set on the basis of values in the columns. If no WHERE clause is specified, all rows of the intermediate result set are selected.

All column references in the search-condition must uniquely identify a column in the intermediate result set defined by the FROM clause or be an outer reference.

Column references must be qualified if more than one column in the intermediate result set has the same name, or if the column is an outer reference.